Coatings, Coating Compositions, and Methods of Delaying Ice Formation

ABSTRACT

A coating includes at least one coating layer containing first particles, second particles, and third particles distributed throughout a cross-linked, continuous polymer matrix. An outer surface of the coating layer includes surfaces of at least first particles extending outward from a top periphery of the polymer matrix. The outer surface exhibits a property of delaying ice formation compared to the coating layer without the first particles. A method includes applying a coating composition in one application step. The one-step coating composition contains first particles, second particles, and third particles in a base containing a polymer. A coating composition includes first particles, second particles, and third particles distributed in a matrix precursor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 14/058,470, filed Oct. 21, 2013, and entitled “Coatings,Coating Compositions, and Methods of Delaying Ice Formation” whichclaims the benefit of priority under 35 U.S.C. §119 to U.S. ProvisionalApp. No. 61/838,605, filed Jun. 24, 2013, entitled “Coatings, CoatingCompositions, and Methods of Delaying Ice Formation,” both of which areherein incorporated by reference.

TECHNICAL FIELD

Compositions and methods herein pertain to coatings, coatingcompositions, and methods, including those related to delaying iceformation, such as in aerospace applications.

BACKGROUND

Accumulation of frost, ice, or snow on aircraft changes airflow overaircraft wings, reducing lift and increasing drag. The accumulationsalso add to the total weight, increasing lift required for takeoff.Accordingly, frost, ice, or snow is normally removed prior to take-off.While in flight, hot engine bleed air, electric blankets, mechanicalboots, or combinations thereof may be used to keep ice off exteriorsurfaces of aircraft. Unmanned Aerial Vehicles (UAV) and rotorcraftcannot operate at certain altitudes due to potential icing of flightsurfaces. The listed measures actively consume energy, add weight, andreduce fuel economy.

On the ground, anti-icing and de-icing fluids in the form of hot glycolsprays are employed. While effective, such fluids generate an addedexpense and may cause gate delays from additional application time. Theglycol fluids may generate added expense for remediation. As a result,different options for removing ice from aircraft are desirable.

SUMMARY

A coating includes at least one coating layer containing first particlessubstantially homogeneously distributed throughout a thickness of across-linked, continuous polymer matrix. The thickness of the polymermatrix extends inward from a top periphery of the polymer matrix to abottom periphery of the polymer matrix. The coating layer also containssecond particles substantially homogeneously distributed throughout thethickness of the polymer matrix. The coating layer further containsthird particles substantially homogeneously distributed throughout thethickness of the polymer matrix. The second particles have a compositiondifferent from the third particles and first particles. An outer surfaceof the at least one coating layer includes surfaces of at least firstparticles extending outward from the top periphery of the polymermatrix. The outer surface exhibits a water contact angle greater than90° and exhibits a property of delaying ice formation compared to thecoating layer without the first particles.

A method includes applying a coating composition in one applicationstep. The one-step coating composition contains first particles, secondparticles, and third particles in a base containing a polymer. Thesecond particles have a composition different from the third particlesand first particles and the third particles having a compositiondifferent from the first particles. The method includes curing theapplied one-step coating composition, forming a cross-linked, continuouspolymer matrix, and forming a layer of a cured coating. First particlesare substantially homogeneously distributed throughout a thickness ofthe cross-linked, continuous polymer matrix, the thickness of thepolymer matrix extending inward from a top periphery of the polymermatrix to a bottom periphery of the polymer matrix. Second particles aresubstantially homogeneously distributed throughout the thickness of thepolymer matrix. Third particles are substantially homogeneouslydistributed throughout the thickness of the polymer matrix. Also, themethod includes forming an outer surface of the cured layer including atleast first particles extending outward from the top periphery of thepolymer matrix. The outer surface exhibits a water contact angle greaterthan 90° and exhibits a property of delaying ice formation compared tothe cured layer without the first particles.

A coating composition includes first particles, second particles, andthird particles distributed in a matrix precursor. The second particlescontain polytetrafluoroethylene and exhibit a particle size range offrom about 100 nm to about 500 nm. The first particles are hydrophobicand contain a material selected from the group consisting of silica,carbon, graphite, fluoropolymers, alumina, ceria, zirconia, titania,zinc oxide, and combinations thereof and exhibit a particle size rangeof less than about 100 nm. The third particles contain a materialselected from the group consisting of calcium silicate, calciumcarbonate, iron oxides, Fe₂O₃, Fe₃O₄, FeOOH, and combinations thereof.The third particles also exhibit a particle size range for their largestdimension of greater than about 250 nm and further stabilize dispersionof the second particles compared to the coating composition without thethird particles.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments or may be combined in yetother embodiments further details of which can be seen with reference tothe following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Some aspects are described below with reference to the followingaccompanying drawings.

FIGS. 1A and 1B are diagrams of an icephobic coating with particlessubstantially homogeneously distributed in a matrix.

FIGS. 2A and 2B and 3A and 3B are coating micrographs taken by ascanning electron microscope (SEM).

FIGS. 4-7 are charts of freezing delay for various evaluated coatingsamples.

FIG. 8 is a functional block diagram summarizing some of therelationships described in further detail herein.

DETAILED DESCRIPTION

The ice reduction measures disclosed herein do not actively consumeenergy and are not single-use applications of materials that also useenergy for each application. Materials described herein do not useexternal energy during operation, avoid the use of glycol ethers, andprovide ice delays over multiple freeze cycles. Consequently, themethods and materials herein may increase aircraft fuel efficiency,reduce use of deicing fluids and their environmental impact, reducein-flight energy consumption used for ice removal, etc. Althoughdiscussed in the context of aviation use, other possible uses arecontemplated, such as on wind turbine blades, in non-aerospacetransportation, and in communications, including on satellite dishes.The material may be formulated as a coating, such as a paint.

Materials described herein are thus passive coatings that use noexternal input of energy to delay ice formation and retain theirefficacy after multiple freeze cycles. Some known coatings purport topossess similar benefits. Even so, none provide the herein describedlevel of dewetting and freezing delay combined with toughness anddurability. The exterior environment associated with aircraft and othertransportation applications (such as, automotive applications) overextended periods may be harsh. Methods and materials herein provide acoating with a cross-linked polymer matrix for toughness and durability.(See FIG. 8, blocks 812 and 802). Also, porous voids and surfaceroughness of the coating exist on a length scale of hundreds ofnanometers to inhibit the wetting of water. (See FIG. 8, blocks 826 and806). Further, a layer of nanoparticles at the coating surface inhibitsnucleation of ice. (See FIG. 8, blocks 834 and 808). Additionally,structural features substantially homogeneously distributed across thecoating thickness present similar performance initially and followingerosion, defined as removing part of the coating, including abrasion andother forms of wear. (See FIG. 8, blocks 820 and 804).

Known coatings may involve a porous foam network created by extractionof colloidal templates to produce some of the benefits indicated above.As used herein, a “templating material” or “template” refers to adiscrete particle that affects the shape of the polymer surrounding it.A templating material, such as solid particles, may be dispersed in acontinuous matrix. The matrix acquires a structure around the templatingmaterial and the structure remains after extraction of the templatingmaterial. Previously, intentional removal of templates was viewed as aprerequisite to creating a sufficient amount of air/water interface toachieve high contact angles and dewetting. The methods and materialsherein do not necessarily involve removal of templating material, butstill achieve high dewetting performance. Observation hereinsurprisingly indicated that the surface roughness and voids that drivehigh contact angle dewetting behavior may be created instead throughselection of certain template morphologies, even when the templatingmaterial remains in the matrix. (See FIG. 8, blocks 816, 824, 832, and826). Additionally, it was surprising that a polymer matrix around solidparticles formed voids rather than coating the particles fully.

A coating includes at least one coating layer containing first particlessubstantially homogeneously distributed throughout a thickness of across-linked, continuous polymer matrix. The thickness of the polymermatrix extends inward from a top periphery of the polymer matrix to abottom periphery of the polymer matrix. The coating layer also containssecond particles substantially homogeneously distributed throughout thethickness of the polymer matrix. The coating layer further containsthird particles substantially homogeneously distributed throughout thethickness of the polymer matrix. The second particles have a compositiondifferent from the third particles and first particles. An outer surfaceof the at least one coating layer includes surfaces of at least firstparticles extending outward from the top periphery of the polymermatrix. The outer surface exhibits a water contact angle greater than90° and exhibits a property of delaying ice formation compared to thecoating layer without the first particles.

By way of example, the second particles may contain a fluoropolymer. Thepolymer matrix may be substantially fluorine-free. A “substantially”fluorine-free continuous polymer matrix refers to some trace amounts offluorine that may be present in the polymer. For example, fluorine mightnot be a primary component of the polymer chain itself, but nonethelesspart of the polymer as a catalyst residue, contaminant, etc.Additionally, the polymer matrix need not incorporate exotic fluorinepolymers, such as fluorinated polyhedral oligomeric silsesquioxanes,that are costly and difficult to manufacture. Known polyhedraloligomeric silsesquioxanes are available under the trade name POSS fromHybrid Plastics, Inc. of Hattiesburg, Md. A substantially fluorine-freecontinuous polymer matrix may still allow the benefits arising from acompletely fluorine-free matrix, which include more choice in selectionof a polymer, no requirement of polymer fluorination measures, and,thus, simplified coating application procedures. The first particles mayalso be substantially fluorine-free, which refers to some trace amountsof fluorine that may be present. A substantially fluorine-free firstparticle may still allow the benefits arising from a completelyfluorine-free first particle.

“Substantially” homogeneously distributed particles refers to somedeviation from a perfect distribution that may occur and still providethe properties of water contact angle and delays in ice formation. Sincethe coating could be formed over an underlying substrate, the “outersurface” of the coating layer refers to a surface of the coating layeroutward or away from the substrate while the “inner surface” refers to asurface of the coating layer inward or toward the substrate. Ahomogeneous distribution is a distribution with a uniform structure orcomposition; for example, a uniform pattern of distributed particles.

The uniform pattern may be single particles spaced equally, multiplegroupings of particles with the groupings spaced equally, or some otherpattern. A substantially homogeneous distribution of each of firstparticles, second particles, and third particles is shown in FIG. 1A.Notably, all of the first particles are not equally spaced, but they arearranged in a substantially uniform pattern and are thus substantiallyhomogeneously distributed. Since “each” of the three different types ofparticles is substantially homogeneously distributed, the firstparticles are individually substantially homogeneously distributed, asare the second and third particles, each type in its own substantiallyuniform pattern. Given the different size scales and other factors (asdiscussed further below), some migration together of the first particleswith the second particles is shown, but does not upset the individualdistribution of each of the three types of particles in their ownrespective substantially uniform pattern. Indeed, such migration maypromote homogeneity (see below).

The coating may be formed from damage-tolerant layers including thecross-linked, polymer matrix with exposed nanoparticles that delay thenucleation of ice. The coating may retain its properties after erosion,such as by abrasion or impacts, and may be sufficiently tough to surviverain erosion from collision with raindrops. The coatings may be formedusing a templating process wherein a matrix precursor or liquid base ismixed with tem plating colloids and nanoparticles, the mixture appliedto a surface, and the matrix precursor cured.

Possible polymers include polyurethanes, polyfluorourethanes, epoxies,carbamates, and polysiloxanes that may be cross-linked in the matrix.The coating may be created from aerospace qualified materials that passdurability testing to allow effective use on aircraft. Examples ofmatrix precursors include DESOTHANE® HS CA8800/B900 available from PPGAerospace in Sylmar, Calif. and ECLIPSE® (ECL-G-7) available from AkzoNobel Aerospace Coatings in Waukegan, Ill., both of which are solventborne gloss polyurethane clear coats. The matrix precursor may contain apolymerizable species and, optionally, a solvent.

Aerospace qualified materials are often resistant to ultraviolet (UV)radiation, as is aliphatic urethane, instead of aromatic urethane, whichis not UV resistant. Aerospace qualified materials may be resistant tocommon materials encountered in aerospace applications, such ashydraulic fluid. While some aerospace qualified materials may bebeneficial in an exterior application on aircraft, other aerospacequalified materials may only be qualified for an interior application onaircraft, such as polysiloxane and carbamate polymers. Interior polymersmight not be exposed to UV radiation or hydraulic fluid, but still helpdelay ice accumulation, such as interior condensation in the form offrost on airplane structures. Rapidly melting frost can create nuisancemoisture in an airplane cabin. One benefit of at least some of thematerials used in the coatings herein includes their compatibility withsome known, aerospace qualified matrix precursors so that re-formulationis unnecessary.

The precursor solution may include solvents that do not adversely reactin formation of the cross-linked network or dissolve/swell thetemplating particles. Examples include water, alcohols (such as,methanol, ethanol, isopropanol, t-butanol), acetone, ketones (such as,methyl ethyl ketone, methyl isobutyl ketone, and methyl amyl ketone),toluene, and t-butyl acetate. A colorant and other additives may beincluded in the precursor solution.

Tem plating materials may create roughness in the polymer matrix withoutremoval of the materials. (See FIG. 8, blocks 824, 832, and 826).Templating materials may have an inorganic composition, such as, CaSiO₃(calcium silicate), CaCO₃ (calcium carbonate), or iron oxides (such as,Fe₂O₃, Fe₃O₄, FeOOH). Furthermore, acicular CaCO₃ particles orneedle-shaped Wollastonite (CaSiO₃) particles may be used. Wollastoniteis a calcium silicate mineral that may contain trace amounts of otherelements. Iron oxides, Fe₂O₃, Fe₃O₄, and FeOOH may also be acicular.

TEFLON® polytetrafluoroethylene (PTFE) may be included as anothertemplating material, such as, mixtures of 100-500 nanometer (nm) TEFLONparticles with the templating materials listed above. DUPONT™ TEFLON®ZONYL® MP 1000 nanoparticles with 200-300 nm diameter are suitable.TEFLON 200 nm particles have been used to induce roughness. (See FIG. 8,blocks 816 and 826). Observation indicated that the TEFLON particlesreduce water infiltration of a coating as compared to coatings withoutTEFLON. (See FIG. 8, blocks 816 and 818). The reduction in waterinfiltration increases durability and increases delay in ice formationcompared to coatings without TEFLON. (See FIG. 8, blocks 818 and 802).However, observation further indicated that using only TEFLON, withoutother listed templating materials, did not yield a substantially uniformdistribution of the TEFLON. (See FIG. 8, blocks 828 and 820). CaCO₃ orWollastonite bears the effect of substantially uniformly distributingTEFLON in a coating composition and, thus, the resulting coating. Whenno CaCO₃ or Wollastonite was used, the TEFLON phase separated during jarmilling.

Templating materials may undergo a surface treatment to increasehydrophobicity prior to incorporation in a coating composition. Possiblesurface treatments include alkylsilanes, fluoroalkylsilanes, phosphonicacids, and carboxylic acids, including fatty acids.

Additional particles in the form of hydrophobic nanoparticles may be inthe coating composition. Possible nanoparticles include silica, carbon,graphite, fluoropolymers, alumina, ceria, zirconia, titania, and zincoxide. Like the templating materials, the nanoparticles may undergo asurface treatment to increase hydrophobicity prior to incorporation inthe coating composition. Possible surface treatments are alkylsilanesand fluoroalkylsilanes. Hydrophobic silica nanoparticles of about 20 nmin size have been observed to inhibit ice nucleation.

As may be appreciated, a variety of examples are contemplated forimplementing the coating introduced above. The third particles may havea composition different from the first particles. Consequently, thefirst particles may contain SiO₂ and the third particles may containCaSiO₃. The fluoropolymer may contain polytetrafluoroethylene. The thirdparticles may exhibit particle sizes greater than the second particles,which may exhibit particle sizes greater than the first particles. Therelevant particle sizes may be the values determined according to knownstandards for testing and measurement of particle sizes appropriate forthe size range and composition of the particles.

In more detail, the third particles may exhibit a particle size rangefor their largest dimension of greater than about 250 nm, such asgreater than about 500 nm, including greater than about 500 nm to about20,000 nm. The third particles may have an anisotropic particle size sothat the largest dimension is from about 2,000 nm to about 10,000 nm,such as from about 3,000 nm to about 6,000 nm. The smallest dimensionmay be from about 100 nm to about 2,000 nm, such as from about 400 nm toabout 600 nm. Accordingly, third particles may exhibit aspect ratios ofat least about 1.5:1, such as from about 1.5:1 to about 10:1.

The second particles may exhibit a particle size range of from about 100nm to about 10,000 nm, such as from about 100 nm to about 500 nm,including from about 200 nm to about 300 nm. The first particles mayexhibit a particle size range of less than about 100 nm, such as lessthan about 50 nm, including from about 5 nm to about 50 nm, for example,from about 15 nm to about 30 nm. In the coating, the coating layer mayexhibit a mass ratio of 1st:2nd:3rd, wherein “1st,” “2nd,” and “3rd” arethe respective proportions of first, second, and third particles, 2ndbeing greater than both 1st and 3rd.

The polymer in the coating need not be hydrophobic and/or icephobic forthe coating to exhibit beneficial properties described herein. As aresult, the polymer matrix may exhibit a water contact angle less than90°. For the coating to nonetheless delay ice formation, the outersurface of the coating layer may include surfaces of at least firstparticles in addition to surfaces of polymer matrix. The outer surfacemay further include surfaces of second particles extending outward fromthe top periphery of the polymer matrix and, perhaps, even thirdparticles.

Exposure of the particles at the outer surface may thus provide thedesired properties without a hydrophobic and/or icephobic polymermatrix. Less restriction on the properties of the polymer matrix allowsgreater flexibility in formulation. (See FIG. 8, blocks 836, 812, and810). Consequently, the coating does not rely on a fluorinated polymermatrix to delay ice formation and the matrix may be substantiallyfluorine free, affording the benefits of a fluorine-free matrixdescribed above. Because first particles, second particles, and thirdparticles are each substantially homogeneously distributed in thepolymer matrix, erosion, such as abrasion or other wear, of the outersurface to form another outer surface may preserve similar properties.The removed polymer matrix and particles merely reveal underlyingparticles that function similarly.

The coating may include a plurality of such coating layers, each withits own outer surface. The outer surface of underlying coating layers isnot necessarily exposed since it may be covered by the inner surface ofan overlying coating layer. The polymer matrix may include polyurethaneor the polymer matrix may meet qualification requirements in accordancewith an aerospace specification or both. The polymer matrix may meetqualification requirements in accordance with specifications for otherapplications, such as use on wind turbine blades.

The second particles may exhibit the property of reducing waterinfiltration compared to an equal mass of the first particles, thirdparticles, or a combination thereof. Water infiltration may becontrolled by reducing porosity. The coating layer may exhibit a curedporosity of at most 70 volume percent (vol %), such as at most 25 vol %,including from about 5 vol % to 25 vol %. Allowing at least someporosity may provide distributed void interfaces, such as air-polymerinterfaces, and promote nanoparticle distribution, as discussed furtherbelow. Interfaces between polymer and particles much larger thannanoparticles may also promote nanoparticle distribution, as discussedfurther below. Despite a porosity value of at most 25 vol %, the outersurface may exhibit a surface roughness on a length scale of from about100 nm to about 5,000 nm (200 microinches), such as from about 100 nm toabout 4,000 nm, as induced by the templating material.

FIG. 1A is a diagram of a portion of a layer 102 of a coating 100. Whilean outer surface 114 of layer 102 is apparent, FIG. 1A is not intendedto depict an inner surface of layer 102 or underlying coating layers.Optional, underlying layers 104 and 106 of coating 100 are shown in FIG.1B formed over substrate 108 and may have the same composition andexhibit the same structural features as layer 102. Layer 102 containsfirst particles substantially homogeneously distributed throughout athickness 110 of the matrix. The matrix is shown in FIG. 1A as acontinuous matrix, which may be substantially fluorine-free andcross-linked. Thickness 110 of the matrix extends inward from a topperiphery 112 of the matrix to a bottom periphery (not shown in detailin FIG. 1A) of the matrix. Layer 102 also contains second particlessubstantially homogeneously distributed throughout thickness 110 of thematrix. Layer 102 further contains third particles substantiallyhomogeneously distributed throughout thickness 110 of the matrix.

It will be appreciated that the first, second, and third particles arenot drawn to scale, but are intended to graphically depict approximateshapes and the feature that the third particles exhibit particle sizesgreater than the second particles, which exhibit particle sizes greaterthan the first particles. The second particles may contain afluoropolymer and may have a composition different from the thirdparticles and the first particles. Additionally, the third particles mayhave a composition different from the first particles. Compositionaldifferences are represented in FIG. 1A with different hatching patternsfor the matrix and the first, second, and third particles. As opposed toa known continuous layer of fluoropolymer in a coating, as in the priorart, the fluoropolymer herein may exist in discrete particles throughoutthe matrix, providing additional benefits beyond the mere hydrophobicityof fluoropolymer materials.

Layer 102 includes an outer surface 114 with surfaces of at least firstparticles extending outward from top periphery 112 of the matrix. InFIG. 1A, outer surface 114 includes surfaces of first particles 114 a,matrix 114 b, and second particles 114 c. Surfaces of second particles114 c of outer surface 114 extend outward from top periphery 112 c ofthe matrix. Surfaces of first particles 114 a of outer surface 114extend outward from top periphery 112 a of the matrix. Surfaces ofmatrix 114 b of outer surface 114 coincide with top periphery 112 b ofthe matrix.

By virtue of the roughness of outer surface 114 and the presence ofhydrophobic first particles and second particles, outer surface 114 mayexhibit a water contact angle greater than 90 degrees. (See FIG. 8,blocks 828 and 806). Outer surface 114 may include surfaces of the firstparticles exhibiting a particle size range of less than about 100 nm,such as from about 5 nm to about 50 nm. As a result, outer surface 114may exhibit a property of delaying ice formation compared to layer 102without the first particles. Such properties may be afforded even in thecircumstance where the matrix exhibits a water contact angle less than90 degrees, including at surfaces of matrix 114 b of outer surface 114.

The polarity (or hydrophobicity) of particles may also contribute todelaying ice formation. Less polar (more hydrophobic) particles maydelay ice formation since ice is polar and formation on a non-polar orless polar surface is energetically less favorable. (See, Conrad, P.;Ewing, G. E.; Karlinsey, R. L.; Sadtchenko, V. J. Chem. Phys. 2005, 122,064709). The tendency of one surface to allow ice formation more readilycompared to another surface has been attributed in some cases to asignificantly larger contact angle for the hydrophobic surface. (SeeFIG. 8, blocks 806 and 808).

Layer 102 may exhibit a cured porosity of at most 70 vol %, such as atmost 25 vol %. Porosity may be present as voids (not shown) in thematrix. The first particles may form a Pickering emulsion with voids.Such an emulsion may include two phases having an interfacetherebetween, for example, an air-liquid interface, ahydrophilic-hydrophobic interface, etc. Solid particles, which adsorbonto the phase interface, may stabilize the emulsion. When oil and waterare mixed, small droplets of oil may form throughout the waterinitially, but later recombine to reduce the energy of the mixture.However, solid particles added to the mixture may bind to the surface ofthe droplet's oil/water interface and stabilize the emulsion by makingthe combination of oil droplets less energetically favorable. Particlesize, shape, and hydrophobicity may further affect stability. In thissense, the particles behave as a “surfactant.”

Voids dispersed throughout the matrix of layer 102 provide an air-liquidinterface to which the first particles may adsorb. Accordingly,uniformly distributed voids may assist in uniformly distributing firstparticles. While porosity resulting from voids in the matrix may assistin distributing first particles, too much porosity reduces strength andincreases water penetration of coating 100. For such reason, someporosity may be desirable, such as, from at least 5 vol % to at most 25vol %.

When the matrix is less hydrophobic than second particles and/or thirdparticles, or perhaps hydrophilic with a water contact angle less than90°, another interface exists in layer 102. First particles may thenadsorb onto the interface between a less hydrophobic matrix and a morehydrophobic particle that is larger than first particles. Since thecoating described herein includes use of a hydrophilic polymer matrixand second and third particles may be treated to increasehydrophobicity, hydrophilic-hydrophobic interfaces may exist which aredispersed throughout the matrix. The dispersed interfaces further assistin uniformly distributing first particles. In the case of TEFLON secondparticles in a polyurethane matrix, the surface polarity of secondparticles does not match well with the polyurethane, so first particles,such as silica, may coat the second particles as a surfactant to reduceflocculation of the second particles, such as by forming a Pickeringemulsion. (See FIG. 8, blocks 830, 822, and 820).

Top periphery 112 of the matrix may also present an air-liquid interfaceto which first particles may migrate. When a coating composition isapplied and the air-liquid interface at top periphery 112 is created,first particles may act as a surfactant, adsorbing to the interface.Such behavior provides first particles extending outward from topperiphery 112 of the matrix. First particles are both adsorbed to thecured matrix and exposed as part of outer surface 114 of layer 102. (SeeFIG. 8, blocks 830 and 834). Second particles exhibiting ahydrophobicity difference from the matrix and a particle size range fromabout 100 nm to about 500 nm may also migrate to top periphery 112.Second particles may then also be adsorbed to the cured matrix andextend outward from top periphery 112 to be exposed as part of outersurface 114. (See FIG. 8, blocks 828 and 806).

In addition to first particles stabilizing second particles in thematrix as an emulsion, observation indicated that third particles alsocontributed to substantially homogeneously distributing second particlesin the matrix. Second particles may be too large to form a Pickeringemulsion with the much larger third particles. Also,polytetrafluoroethylene second particles likely will not preferentiallygo into either a water or oil phase. That is, while they arehydrophobic, they might also be lipophobic and/or oleophobic. Withoutthe presence of third particles, second particles were observed toseparate from the matrix liquid phase during mixing by jar milling.Without being limited to any particular theory, suitable third particlesmay act as a flow reducer and thickener in a coating composition. Thehigh aspect ratio of needle-shaped, acicular, cylindrical, or similarparticles may produce a viscosity increase and screen migration ofparticles in the liquid phase of a coating composition. Accordingly,third particles might assist in reducing agglomeration of secondparticles and in keeping them dispersed. (See FIG. 8, blocks 814 and820).

Respective substantially homogeneous distribution of first, second, andthird particles throughout the matrix may provide the benefit ofpreserving hydrophobic and icephobic properties of outer surface 114.When first particles and second particles are removed from outer surface114, the properties may remain at least to some extent. Erosion of layer102 may remove portions of outer surface 114, including first particlesand second particles, exposing underlying matrix and other firstparticles and second particles. Removal of particles and the matrixtends to occur at the weakest points in layer 102, namely at matrix-voidinterfaces and matrix-particle interfaces. As a result, since firstparticles may be adsorbed at void interfaces, erosion tends to exposeparticles underlying outer surface 114 that migrated to the air-liquidinterface of voids. Particles that migrated to thehydrophobic-hydrophilic interfaces around second and/or third particlesmight similarly be exposed by erosion. In this manner, even though thematrix may be hydrophilic, another hydrophobic outer surface may beformed after erosion. Such another outer surface may also exhibit theproperty of delaying ice formation.

A method includes applying a coating composition in one applicationstep. The one-step coating composition contains first particles, secondparticles, and third particles in a base containing a polymer. Thesecond particles have a composition different from the third particlesand first particles and the third particles have a composition differentfrom the first particles. The method includes curing the appliedone-step coating composition, forming a cross-linked, continuous polymermatrix, and forming a layer of a cured coating.

First particles are substantially homogeneously distributed throughout athickness of the cross-linked, continuous polymer matrix, the thicknessof the polymer matrix extending inward from a top periphery of thepolymer matrix to a bottom periphery of the polymer matrix. Secondparticles are substantially homogeneously distributed throughout thethickness of the polymer matrix. Third particles are substantiallyhomogeneously distributed throughout the thickness of the polymermatrix. Also, the method includes forming an outer surface of the curedlayer including at least first particles extending outward from the topperiphery of the polymer matrix. The outer surface exhibits a watercontact angle greater than 90° and exhibits a property of delaying iceformation compared to the cured layer without the first particles.

In addition to the outer surface properties of a cured layer formed fromthe coating compositions described herein, one-step application isanother benefit. Some prior art forms multiple layers of differingcomposition to complete a coating or treat a coating after applicationto imbue certain properties. In contrast, materials of coatingcompositions herein inherently possess desired properties in theiras-applied state without further layers, treatment, etc. Curing merelysolidifies the structural features.

By way of example, the cured layer may exhibit a porosity of at most 25vol %. The cured layer may be formed over an exterior surface of anaircraft. The method may further include forming a plurality of thelayers of the cured coating by repeating the application of the coatingcomposition over the exterior surface and curing the repeatedly appliedone-step coating composition. The second particles may contain afluoropolymer and the polymer matrix may be substantially fluorine-free.The particles may exhibit the size ranges described elsewhere herein.The outer surface may exhibit the surface roughness described elsewhereherein.

Applying the coating composition may include spray applying using knowntechniques. Known paint on methods or appliques may be used instead,though a benefit exists in compatibility with known spray equipment forefficient coverage of large areas. Curing the coating composition mayinclude completing the cross-linking or completing the cross-linkingsufficiently for the intended use of the coating. The polymer matrix mayexhibit a water contact angle less than 90° and the outer surface of thecured layer may further comprise polymer matrix. The second particlesmay be selected to exhibit the property of reducing water infiltrationcompared to an equal mass of the first particles, third particles, or acombination thereof. Eroding the outer surface forms another outersurface, the other outer surface exhibiting a water contact anglegreater than 90° and a property of delaying ice formation compared tothe cured layer without the first particles.

A coating composition includes first particles, second particles, andthird particles distributed in a matrix precursor containing apolymerizable species and, optionally, a solvent. The second particlescontain polytetrafluoroethylene and exhibit a particle size range offrom about 100 nm to about 500 nm. The first particles are hydrophobicand contain a material selected from the group consisting of silica,carbon, graphite, fluoropolymers, alumina, ceria, zirconia, titania,zinc oxide, and combinations thereof and exhibit a particle size rangeof less than about 100 nm. The third particles contain a materialselected from the group consisting of calcium silicate, calciumcarbonate, iron oxides, Fe₂O₃, Fe₃O₄, FeOOH, and combinations thereof.The third particles also exhibit a particle size range for their largestdimension of greater than about 250 nm and further stabilize dispersionof the second particles compared to the coating composition without thethird particles.

By way of example, the first particles may reduce flocculation of thesecond particles and stabilize dispersion of the second particles. Asone possibility, the first particles may form a Pickering emulsion withthe second particles. The matrix precursor may be substantiallyfluorine-free. The first particles may contain SiO₂ and exhibit aparticle size range of from about 5 nm to about 50 nm. The thirdparticles may contain CaSiO₃ and exhibit a particle size range for theirlargest dimension of greater than about 500 nm to about 20,000 nm. Thematrix precursor may meet qualification requirements in accordance withan aerospace specification. In more detail, the coating composition mayexhibit a mass ratio of B:1st:2nd:3rd, wherein “B” is the proportion ofsolids in the matrix precursor and “1st,” “2nd,” and “3rd” are therespective proportions of first, second, and third particles, B beingless than the sum of 1st, 2nd, and 3rd and 2nd being greater than both1st and 3rd.

Examples Experimental.

A variety of sample coatings was prepared and is summarized below inTable 1. Mass ratios of B:1st:2nd:3rd indicated B=base, 1st=silica,2nd=TEFLON, and 3rd=CaCO₃ or CaSiO₃. For samples using PPG DESOTHANECA8800/6900 with a 1:1:4:2 mass ratio, such as sample 130, they weremade as follows: Silica (2 grams (2 g)), TEFLON (8 g), CaCO₃ or CaSiO₃(4 g), and PPG CA8800F Thinner (23 g) were added to a 50 milliliter (mL)centrifuge tube then briefly vortexed to homogenize the mixture.DESOTHANE CA8800/6900 Base (1.36 g) and PPG CA8800Z Activator (0.76 g),which contain a total of 2 g of solids, were added to the suspension offiller (particles) and briefly vortexed. Samples with other mass ratiosand/or fillers, such as samples 162, 163, 167, and 168, were preparedsimilarly.

Also, for samples using Akzo Nobel ECL-G-7 with 1:1:4:2 mass ratio, suchas samples 126 and 137, they were made as follows: Silica (2 g), TEFLON(8 g), CaCO₃ or CaSiO₃ (4 g), and Akzo Nobel TR-109 Thinner (22 g) wereadded to a 50 mL centrifuge tube then briefly vortexed to homogenize themixture. ECL-G-7 Base (1.92 g) and Akzo Nobel PC-233 Activator (1.15 g),which contain a total of 2 g of solids, were added to filler suspensionand briefly vortexed.

The mixture was then weighed and mixed for 15 minutes with a planetarycentrifugal mixer available from THINKY USA, Inc. in Laguna Hills,Calif. The mixture was weighed again and solvent was added to restorethe original mass.

Coupons painted with an aerospace white topcoat (PPG DESOTHANE CA8000BAC70846 white) as control specimens were prepared by spraying withSur-Prep AP-1 adhesion promoter manufactured by AndPak, Inc. in MorganHill, Calif. and allowed to dry for 30 minutes. Then the paint mixturewas placed in a DeVilbiss (in Glendale Heights, Ill.) gravity spray gunand applied using an air pressure of 10 pounds per inch² (psi) on thecoupons. The painted coupons were left in a fume hood for at least 10hours and then either cured in an 80° C. oven for 4 hours or a 50° C.oven for 16 hours.

Materials.

Materials used to create coatings may be further described as follows.PPG DESOTHANE CA8800/6900 is a two-part polyurethane aerospaceclear-coat paint. Akzo Nobel ECLIPSE ECL-G-7 is a two-part polyurethaneaerospace clear-coat paint.

Hexamethyldisilazane (HMDZ) treated silica nanoparticles are 22 nm indiameter and were used to inhibit icing. The particle diameter issimilar to the size of ice nuclei and is believed to disrupt them.Particles of about this size are theorized to reduce the nucleation rateof ice based on both their hydrophobic character and the high curvatureforced on the ice nuclei during heterogeneous nucleation on the particlesurface. (See, Conrad, P.; Ewing, G. E.; Karlinsey, R. L.; Sadtchenko,V. J. Chem. Phys. 2005, 122, 064709). Heterogeneous nucleation refers tonucleation on a surface other than ice, while homogeneous nucleationrefers to nucleation on ice already formed. The HMDZ treated silicananoparticles were easily incorporated into the DESOTHANE clear coatsystem. Polymer solids-to-silica mass ratios of 1:1, 2:1, and 4:2 wereused in experiments because these ratios were found to increaseinhibition of ice nucleation without coating embrittlement inpreliminary testing.

BYK LP-X22325 (formerly LP-X21261) is a hydrophobic colloidal silicawith an average particle size of 20 nm.

SUPER-PFLEX® 100 CaCO₃ available from Specialty Minerals in Bethlehem,Pa. is a star shaped, stearate modified (1 weight percent (wt %)),hydrophobic filler that creates surface roughness through porosity.Coatings based on this material were favored in preliminary testing butwere supplanted by more durable coatings containing Wollastonite. TheSUPER-PFLEX material was utilized in one of the preferred coatings.

SUPER-PFLEX 200 CaCO₃ is a star shaped, stearate modified (2 wt %),hydrophobic filler that creates surface roughness through porosity. Thiscoating has a stearate density twice as high as SUPER-PFLEX 100.Coatings based on this material were not superior to SUPER-PFLEX 100based coatings and were not preferred.

Wollastonite CaSiO₃ are needle-shaped particles less than 10 micrometer(μm) in length available from NYCO Minerals Inc. in Willsboro, N.Y. inthree variants that have the same size and aspect ratio. CaSiO₃ isattractive because of the variety of morphologies, surface treatments,and robustness against degradation from exposure to acids, bases, orsolvents. Wollastonite 1250WC (NYCO Minerals M1250 Wollastocoat 20804)is surface modified with hydrophobic surface groups to control polarityand amine groups that chemically bond with isocyanates used inpolyurethanes. 1250WC material was in the highest performing coatings.Wollastonite 1250AS (NYCO Minerals M1250 AS Wollastocoat) is surfacemodified with amine groups that chemically bond with isocyanates used inpolyurethanes. Coatings containing this filler had slightly shorter icenucleation delays than coatings containing 1250WC, probably because oftheir more hydrophilic nature. Wollastonite 1250 is particles withoutany surface modification. These particles do not resist waterinfiltration after multiple freezing cycles and were not used inpreferred coatings.

NYCO Minerals Inc. Wollastonite Aspect 3000 WC20804 is a CaSiO₃particulate filler with a higher aspect ratio than Wollastonite 1250WC.It has an average particle size of 7 microns and a surface area of 2.2meter²/g. This material is coated with amine and hydrophobic chemicalgroups.

DUPONT™ TEFLON® ZONYL® MP 1000 nanoparticles are 200-300 nm in diameterand were added to the coatings to maintain surface roughness whiledecreasing water infiltration. TEFLON was used in preferred coatingswith a 1:1:4:2, 2:1:4:2, or 4:2:4:2 (polyurethane: silica: TEFLON:(Wollastonite or CaCO₃)) solids mass ratio.

Results.

The freezing point delay for first observation of ice at −5° C. forcoatings that contained 1:1:2:4 DESOTHANEpolyurethane:silica:TEFLON:SUPER-PFLEX 100 CaCO₃ (solids mass ratio)dropped from over 100 seconds to less than 30 seconds after abrasion.Inspecting the coating with SEM (FIGS. 2A and 2B, top and cut away)showed a TEFLON and silica rich surface and the star shaped CaCO₃present and no TEFLON detected subsurface. The top surface was rich insilica nanoparticles and TEFLON while the interior showed less of thosefillers and more CaCO₃. This effect was not unique for CaCO₃ basedcoatings, but was also observed when Wollastonite was used. It wastheorized that the deteriorated wetting resistance after abrasion wasdue to low amounts of TEFLON and silica present in the body of thecoating due to insufficient mixing.

To increase the freezing delay after abrasion, the mixing of theformulations was increased to homogenize throughout the coating thedistribution of fillers that affect icing. The fillers were firstcombined with solvent, jar milled for 3 to 12 hours to disperse them inthe solvent, mixed with polyurethane coating material, and mixed in aThinky centrifugal mixer. The end result was a more uniform top surfaceas shown in FIGS. 3A and 3B. Top SEM images of coatings without (FIG.3A) and after (FIG. 3B) premixing of fillers in a jar mill followed byThinky mixing of the formulation were produced. Increased mixing of theformulation resulted in more uniformly distributed TEFLON and silica atthe surface as well as longer freezing delays.

The −5° C. freezing delay for first observation of ice from a 1:1:2:4DESOTHANE polyurethane:silica:TEFLON:SUPER-PFLEX 100 CaCO₃ (solids massratio) coating increased to almost 300 seconds from 100 seconds as aresult of more mixing. However, after abrasion, the freezing delaydecreased to 30 seconds over three freezing cycles. Reduction infreezing delay after abrasion was also observed for coatings based onWollastonite fillers. SEM showed there still were significantly reducedamounts of TEFLON and silica in the body of the coatings.

Colloidal silicas pre-dispersed in solvents and available from BYK andNissan Chemical for the solid HMDZ fumed silica were then substitutedunder the hypothesis that the already suspended colloidal silica woulddisperse better in the formulations. The result of severely decreasedfreezing delays after abrasion regardless of silica type was observed.FIG. 4 shows −5° C. freezing delays for initial observation of ice frommultiple freezing cycles for coatings containing different silicananoparticles, TEFLON nanoparticles, and SUPER-PFLEX 100 CaCO₃ in a1:1:2:4 DESOTHANE polyurethane:silica:TEFLON:SUPER-PFLEX 100 CaCO₃solids weight ratio. The type of silica is shown in the legend. Thecoatings showed a significant decrease in freezing delay after abrasion.

Because SEM showed a low level of TEFLON inside the coatings, theproportion of TEFLON in the coating was increased to a 1:1:4:2polyurethane:silica:TEFLON:(CaCO₃ or Wollastonite) solids weight ratio.This resulted in increased freezing delay post-abrasion results. HigherTEFLON mass fraction formulations are described in Table 1 along withtheir performance results, such as, contact angle, contact anglehysteresis, freezing delay pre- and post-abrasion, the change infreezing delay after multiple cycles, and the hardness of the coatingfrom a Gardco/Wolff Wilborn Pencil Test.

Samples 162, 163, 167, and 168 were prepared with a greater massfraction of polymer (4 polymer:2 silica:4 TEFLON:2 CaSiO₃ mass ratio)and evaluated. Performance results, including freezing delay pre- andpost-abrasion, are shown in Table 1. FIG. 7 shows freezing point delaythrough three freeze cycles for both pre- and post-abraded samples.

Sample 162 pore volume, measured using mercury porosimetry, was 12.6 vol% using pores less than 9,000 nm. Using pores less than 32,000 nmyielded a pore volume of 20.3 vol %, which could be considered a maximumpossible porosity. The sample was prepared by spraying the coating ontoglass, scraping the coating off the glass, and then analyzing thescraped-off coating. As the pore size cutoff increases, spacing betweenportions of the scraped-off coating might be included in the measuredvalue. Accordingly, actual porosity of the coating itself may be lower.

As a comparison to sample 162, sample 212 was prepared with identicalcomposition except that TEFLON was replaced with additional CaSiO₃ ofequal mass. That is, it contained 4:2:0:6 solids mass ratio of DESOTHANEpolyurethane:hexamethyldisilazane SiO₂:TEFLON:Wollastonite 1250WC.Sample 212 pore volume, measured using mercury porosimetry, was 7.7 vol% using pores less than 9,000 nm. Using pores less than 32,000 nmyielded a pore volume of 13.2 vol %, which could be considered a maximumpossible porosity. The porosity determination sample was prepared byspraying the coating onto glass, scraping the coating off the glass, andthen analyzing the scraped-off coating. As the pore size cutoffincreases, spacing between portions of the scraped-off coating might beincluded in the measured value. Accordingly, actual porosity of thecoating itself may be lower.

Despite the lower porosity, pre-abrasion freeze delay testing on sample212 showed much shorter delays until ice formation on the second andthird freezing cycles. This result suggested that water infiltrated intothe coating, changed the polarity, and accelerated ice nucleationrelative to the starting dry coating. By comparison, sample 162, whichcontained TEFLON maintained the freezing delays over three cycles.Sample 162 had freezing delays at −5° C. of (1st, 2nd, 3rd freezingcycle): 401+/−46 sec, 427+/−28 sec, 383+/−39 sec. Sample 212 hadfreezing delays at −5° C. of (1st, 2nd, 3rd freezing cycle): 363+/−84sec, 48+/−9 sec, 86+/−3 sec.

Notably, the sample with higher porosity (162) nonetheless displayedevidence of reduced water infiltration compared to the sample with lowerporosity (212). Without being limited to any particular theory, TEFLONbeing more hydrophobic than the silica and Wollastonite was believed tobe the cause. If TEFLON lines the pores of sample 162, capillarypressure may be greater against infiltration and less water may seep incompared to sample 212. Even if there are more pores or a greater porevolume, water could have a greater capillary pressure to overcome toenter the TEFLON lined pores. Capillary pressure pulls water into poreslined with hydrophilic materials and pushes water out of pores linedwith hydrophobic materials. Consequently, TEFLON particles exposedwithin pores may reduce water infiltration into pores.

In the formula column of Table 1 PPG is DESOTHANE HS CA8800/6900polyurethane clear coat, AN is Akzo Nobel ECLIPSE G-7 polyurethane clearcoat, S100 is SUPER-PFLEX 100 CaCO₃ (calcium carbonate), S200 isSUPER-PFLEX 200 CaCO₃, WC is Wollastonite 1250WC, AS is Wollastonite1250AS, 1250 is Wollastonite 1250, A3000 is Wollastonite Aspect 3000WC20804, HMDZ silica is Hexamethyldisilazane SiO₂, and BYK is BYKLP-X21261 hydrophobic colloidal silica. For purposes of comparison, atopcoat sample has an initial freezing delay at −5° C. of ˜30 secondsbefore abrasion and ˜10 seconds after abrasion. “Pins” indicates failureof a 50 μL water droplet to roll as the surface was tilted to 90°. Somecoatings have missing measurements because they were not suitable forfurther evaluation based on initial results.

TABLE 1 Abraded % delay % delay Contact Roll retained Abraded retainedContact angle off −5° C. after −5° C. after Formula & Sample anglehysteresis angle freezing cycling freezing cycling mass ratio number(deg) (deg) (deg) delay (s) (#cycles) delay (s) (#cycles) Hardness1:1:4:2 118 124  pins 243 ± 247 16% PPG:HMDZ (2) SiO₂: TEFLON:S1001:1:4:2 119 120° 14° pins 396 ± 105 43% 80 ± 49 61% 5B PPG:HMDZ (3) (2)SiO₂: TEFLON:WC 1:1:4:2 123 145° 23° 15° 160 ± 74  96% 30 ± 6  136%  2BAN:HMDZ (3) (3) SiO₂: TEFLON:WC 1:1:4:2 126 133° 10° pins 407 ± 211 44%75 ± 30 138%  F AN:HMDZ (3) (3) SiO₂: TEFLON:S100 1:1:4:2 129 133°  9°40° 266 ± 31  13% <9B   PPG:HMDZ (3) SiO₂: TEFLON:1250 1:1:4:2 130 142°13° 30° 171 ± 113 40%  92 ± 104 100%  F PPG:HMDZ (3) (3) SiO₂: TEFLON:AS1:1:4:2 134 150° 10°  5° 45 ± 22 73% <9B   AN:HMDZ (2) SiO₂: TEFLON:12501:1:4:2 135 149°  5°  5° 283 ± 133 17% 2B PPG:HMDZ (3) SiO₂: TEFLON:S2001:1:4:2 137 140°  7° 30° 391 ± 84  26% 48 ± 37 82% F AN:HMDZ (3) (3)SiO₂: TEFLON:AS 2:1:4:2 139 150°  7° 10° 229 ± 79  56% 78 ± 37 73% 1HPPG:HMDZ (3) (3) SiO₂: TEFLON:WC 2:1:4:2 140 110°  5° pins 304 ± 22  25%195 ± 89  55% 1H AN:HMDZ (3) (3) SiO₂: TEFLON:WC 4:2:4:2 162 104°  4 pins 410 ± 46  94% 246 ± 102 50% 4H PPG:HMDZ (3) (3) SiO₂: TEFLON:WC4:2:4:2 163  95° 10  pins >500 100%  249 ± 101 45% 4H PPG:BYK (3) (3)SiO₂: TEFLON:WC 4:2:4:2 167 140°  5°  5° >500 100%  74 ± 56 174%  4HPPG:HMDZ (3) (3) SiO₂: TEFLON:A3000 4:2:4:2 168 100°  4° 60° >500 63%197 ± 127 188%  4H PPG:BYK (3) (3) SiO₂: TEFLON:A3000

Conclusions.

The freezing delay of water on coatings made from DESOTHANE mixed withCaCO₃ degraded significantly over multiple cycles.

Coatings containing Wollastonite 1250 did not maintain sufficientfreezing delays for water with multiple freezing cycles. This wasprobably due to the hydrophilic surface.

ECLIPSE polyurethane mixed with HMDZ silica, TEFLON, and SUPER-PFLEXCaCO₃ showed moderately increased contact angles, increased maintenanceof freezing delays pre- and post-abrasion, and increased hardnesscompared to coatings with the same mass ratio and silica type containingWollastonite 1250WC. Sample 126 had a higher hardness rating compared to135 and was used for testing shown in FIG. 5.

Samples containing a 2:1:4:2 polyurethane:silica:TEFLON:Wollastonite1250WC solids mass ratio had increased hardness as well as icenucleation inhibition pre- and post-abrasion after multiple cyclescompared to coatings with less polymer, i.e. 1:1:4:2 mass ratiocoatings. Sample 139 had an increased contact angle. Both samples 139and 140 were used for testing shown in FIG. 5.

Samples containing a 1:1:4:2 polyurethane:silica:TEFLON:Wollastonite1250WC solids mass ratio had insufficient hardness ratings. Theadditional polymer in samples 139 and 140 probably increased thecohesiveness of the coating and resulted in higher hardness.

Samples containing a 4:2:4:2 polyurethane:silica:TEFLON:Wollastonite1250WC or A3000 increased the mass fraction of polymer in the coatingscompared to coatings with less polymer, i.e. 1:1:4:2 or 2:1:4:2. Thegreater amount of polymer was shown to increase damage tolerance. Also,ice nucleation inhibition increased for pre- and post-abrasion aftermultiple cycles.

Samples based on Wollastonite 1250AS had increased hardness, contactangles, and freezing delays compared to coatings with the same massratio and silica type that contain Wollastonite 1250WC. Sample 130 wasused for testing shown in FIG. 5 because it was one of the two bettersamples based on DESOTHANE.

The pre- and post-abrasion −5° C. freezing delays to the time for firstobservation of 3 to 5 droplets of ice for three freezing cycles areshown in FIG. 5 for samples 126, 130, 137, 139, and 140. The samplesperformed better than pre- and post-abraded aircraft white topcoat (PPGDESOTHANE HS CA8000 BAC 70846 white).

The time to initial observation of ice was measured at −10° C. forsamples 126, 130, 137, 139, and 140 as shown in FIG. 6. The samples wentthrough three freezing cycles as indicated by the legend on the graph.The samples performed better than an identical aluminum panel paintedwith aircraft white topcoat (PPG DESOTHANE HS CA8000 BAC 70846 white).

FIG. 7 shows time until initial observation of ice at −5° C. for thehigher polymer mass fraction samples of Table 1. The samples wentthrough three freezing cycles as indicated by the legend on the graph.The times until freezing for the as-made and abraded samples with a4:2:4:2 solids mass ratio were significantly longer than the times forlower polymer content samples in FIG. 5. Furthermore, the hardness ofthe higher polymer mass fraction samples was greater than the lowerpolymer mass fraction samples, which may result in a more durablecoating.

In compliance with the statute, the embodiments have been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the embodiments are not limited tothe specific features shown and described. The embodiments are,therefore, claimed in any of their forms or modifications within theproper scope of the appended claims appropriately interpreted inaccordance with the doctrine of equivalents.

What is claimed is:
 1. A method comprising: applying a coatingcomposition in one application step, the one-step coating compositioncontaining first particles, second particles, and third particles in abase containing a polymer, the second particles having a compositiondifferent from the third particles and first particles, and the thirdparticles having a composition different from the first particles;curing the applied one-step coating composition, forming a cross-linked,continuous polymer matrix, and forming a layer of a cured coating, thecured layer exhibiting a porosity of at most 25 vol %; substantiallyhomogeneously distributing first particles throughout a thickness of thecross-linked, continuous polymer matrix, the thickness of the polymermatrix extending inward from a top periphery of the polymer matrix to abottom periphery of the polymer matrix; substantially homogeneouslydistributing second particles throughout the thickness of the polymermatrix; substantially homogeneously distributing third particlesthroughout the thickness of the polymer matrix; and forming an outersurface of the cured layer including surfaces of at least firstparticles extending outward from the top periphery of the polymermatrix, the outer surface exhibiting a water contact angle greater than90° and exhibiting a property of delaying ice formation compared to thecured layer without the first particles.
 2. The method of claim 1wherein the cured layer is formed over an exterior surface of anaircraft.
 3. The method of claim 1 wherein the second particles comprisea fluoropolymer, the polymer matrix is substantially fluorine-free, andthe method further comprises forming a plurality of the layers of thecured coating by repeating the application of the coating compositionover the exterior surface and curing the repeatedly applied one-stepcoating composition.
 4. The method of claim 1 wherein the thirdparticles exhibit a particle size range for their largest dimension ofgreater than about 500 nm to about 20,000 nm, the second particlesexhibit a particle size range of from about 100 nm to about 500 nm, thefirst particles exhibit a particle size range of from about 5 nm toabout 50 nm, and the outer surface exhibits a surface roughness on alength scale of from about 100 nm to about 5,000 nm.
 5. The method ofclaim 1 wherein the applying comprises spray applying.
 6. The method ofclaim 1 wherein the polymer matrix exhibits a water contact angle lessthan 90°, the outer surface further comprises surfaces of polymermatrix, and the second particles exhibit the property of reducing waterinfiltration compared to an equal mass of the first particles, thirdparticles, or a combination thereof.
 7. The method of claim 1 furthercomprising eroding the outer surface and forming another outer surface,the other outer surface exhibiting a water contact angle greater than90° and a property of delaying ice formation compared to the cured layerwithout the first particles.
 8. The method of claim 1 wherein thepolymer matrix is substantially fluorine-free and the third particleshave a composition different from the first particles.
 9. The method ofclaim 1 wherein the first particles comprise SiO₂, the fluoropolymercomprises polytetrafluoroethylene, and the third particles compriseCaSiO₃.
 10. The method of claim 1 wherein the third particles exhibitparticle sizes greater than the second particles, which exhibit particlesizes greater than the first particles, the cured layer exhibits a massratio of 1st:2nd:3rd, wherein “1st,” “2nd,” and “3rd” are the respectiveproportions of first, second, and third particles, 2nd being greaterthan both 1st and 3rd, and the third particles exhibit aspect ratios ofat least about 1.5:1.
 11. The method of claim 10 wherein the thirdparticles exhibit a particle size range for their largest dimension ofgreater than about 250 nm, the second particles exhibit a particle sizerange of from about 100 nm to about 10,000 nm, and the first particlesexhibit a particle size range of less than about 100 nm.
 12. The methodof claim 1 wherein the outer surface further comprises surfaces ofsecond particles extending outward from the top periphery of the polymermatrix, the polymer matrix exhibits a water contact angle less than 90°,and the outer surface further comprises polymer matrix.
 13. The methodof claim 3 wherein the polymer matrix comprises polyurethane.
 14. Themethod of claim 1 wherein the second particles exhibit the property ofreducing water infiltration compared to an equal mass of the firstparticles, third particles, or a combination thereof.
 15. The method ofclaim 1 wherein the cured layer exhibits a cured porosity of from atleast 5 vol % to at most 25 vol % substantially homogeneouslydistributed throughout the thickness of the polymer matrix.
 16. Themethod of claim 15 wherein the outer surface exhibits a surfaceroughness on a length scale of from about 100 nm to about 5,000 nm. 17.A coating composition comprising: first particles, second particles, andthird particles distributed in a matrix precursor containing apolymerizable species and, optionally, a solvent; the second particlescontaining polytetrafluoroethylene and exhibiting a particle size rangeof from about 100 nm to about 500 nm; the first particles beinghydrophobic, containing a material selected from the group consisting ofsilica, carbon, graphite, fluoropolymers, alumina, ceria, zirconia,titania, zinc oxide, and combinations thereof, exhibiting a particlesize range of less than about 100 nm, reducing flocculation of thesecond particles, and stabilizing dispersion of the second particles;and the third particles containing a material selected from the groupconsisting of calcium silicate, calcium carbonate, iron oxides, Fe₂O₃,Fe₃O₄, FeOOH, and combinations thereof, exhibiting a particle size rangefor their largest dimension of greater than about 500 nm, and furtherstabilizing dispersion of the second particles compared to the coatingcomposition without the third particles.
 18. The coating composition ofclaim 17 wherein the matrix precursor is substantially fluorine-free.19. The coating composition of claim 17 wherein the first particlescomprise SiO₂ and exhibit a particle size range of from about 5 nm toabout 50 nm, the third particles comprise CaSiO₃, exhibit aspect ratiosof at least about 1.5:1, and exhibit a particle size range for theirlargest dimension of greater than about 500 nm to about 20,000 nm. 20.The coating composition of claim 17 exhibiting a mass ratio ofB:1st:2nd:3rd, wherein “B” is a proportion of solids in the matrixprecursor and “1st,” “2nd,” and “3rd” are the respective proportions offirst, second, and third particles, B being less than the sum of 1st,2nd, and 3rd and 2nd being greater than both 1st and 3rd.